The present invention relates to a vortex flowmeter and, particularly, to a vortex flowmeter for air to be supplied to an internal combustion engine, which includes low pass and high pass filters for removing noise components and beat components from vortex a signal so that the vortex frequency can be detected exactly with high response.
FIG. 13 is a block circuit of an example of a conventional vortex flowmeter of this type which is disclosed in Japanese Patent Publication No. 56415/1983. In FIG. 13, an ultrasonic vibrator 4 and an ultrasonic receiver 5 are disposed in facing relation through a flowmeter 1 having a vortex generating member 2 and the ultrasonic vibrator 4 is driven by an ultrasonic oscillation circuit 6 such that ultrasonic waves therefrom propagate across a flow of Karman vortexes 3 produced in a downstream side of the vortex generating member 2.
Ultrasonic waves traversing the Karman vortex flow 3 are phase-modulated thereby and received by the ultrasonic receiver 5. The received signal is shaped by the waveform shaping circuit 8 and supplied to a phase comparator 9.
On the other hand, an output of the ultrasonic oscillator circuit 6 is supplied to a voltage controlled phase deviation circuit 7 having functions of controlling only phase deviation angle while keeping a high stability of ultrasonic oscillation frequency signal.
The output of the ultrasonic oscillation circuit 4 which is phase-deviated by the voltage controlled phase deviation circuit is supplied to the phase comparator 9.
The phase comparator 9, the ultrasonic oscillation circuit 6, the voltage controlled phase deviation circuit 7 and a loop filter 10 constitute a phase synchronizing loop. A reference numeral 11 depicts a low-pass filter.
The phase comparator 9 compares, in phase, the output of the waveform shaping circuit 8 with the output of the voltage controlled phase deviation circuit 7 and supplies a result of comparison to the loop filter 10 in which undesired frequency components are removed.
The voltage controlled phase deviation circuit 7 also responds to an output voltage of the loop filter 10 to control a phase deviation angle of the output signal of the ultrasonic oscillation circuit 6 and the phase comparator 9 is supplied with an output of the voltage controlled phase deviation circuit 7, the latter output being synchronized with the ultrasonic receiving signal. As a result, the output of the low pass filter 11 becomes a phase demodulated output.
In this case, however, the vortex frequency is disturbed by high frequency noise and/or low frequency beat dependent upon flow conditions of the fluid to be detected.
In order to solve this problem, Japanese Patent Publication No. 15045/1983 proposes another Karman vortex flowmeter for an automobile. In this Karman vortex flowmeter, a vortex signal produced corresponding to flow rate or flow speed of the fluid is amplified by an amplifier and filtered by a frequency variable filter whose pass band is controlled corresponding to an information of running conditions of an engine, so that only a true vortex frequency is detected.
In Japanese Utility Model Publication No. 18332/1984 a vortex signal produced is detected by a piezo sensor and, after being converted into an a.c. signal voltage, filtered by a low-pass filter which is an active filter composed of an operational amplifier and an impedence element. When an output of the operational amplifier exceeds a predetermined level, a filter function of the filter is removed.
Further, in Japanese Patent Publication No. 24363/1984, first and second sensors are provided on a Karman vortex generator and outputs of the sensors are supplied to first and second converter/amplifiers, respectively. An addition or subtraction of outputs of the first and the second converter/amplifiers is performed by an operation device whose output is filtered by a filter circuit having low-pass characteristics. An output of this filter circuit is converted into a pulse width signal so that, when the output signal of the operation device exceeds a predetermined level, the filter characteristic of the filter circuit is lost.
That is, in the last two cases, S/N ratio is maintained by inserting the filter function into the circuit, intermittently.
However, in the case of 15045/1983, the frequency control of the frequency variable filter circuit is performed by a control system of an engine. Therefore, there are problems that the control system becomes complicated and the flowmeter itself cannot function independently.
In each of 18332/1984 and 24363/1984, when the output of the operational amplifier constituting a filter exceeds the predetermined level, a switching operation is required to remove the filter characteristics. Therefore, it is impossible to obtain a sufficient S/N ratio.